Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Customer revenues	$ 918.8	$ 861.5	$ 3,377.7	$ 3,320.1
Other revenues	40.6	41.0	113.3	150.6
Total Operating Revenues	959.4	902.5	3,491.0	3,470.7
Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	463.6	454.3	2,160.6	2,224.9
Other revenues	8.7	15.8	21.8	79.5
Total Operating Revenues	472.3	470.1	2,182.4	2,304.4
Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	455.0	407.0	1,216.5	1,094.6
Other revenues	23.9	25.2	68.4	71.1
Total Operating Revenues	478.9	432.2	1,284.9	1,165.7
Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.2	0.2	0.6	0.6
Other revenues	8.0	0.0	23.1	0.0
Total Operating Revenues	8.2	0.2	23.7	0.6
Residential
Disaggregation of Revenue [Line Items]
Customer revenues	483.0	471.7	1,896.3	1,929.6
Residential \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	304.3	306.9	1,456.5	1,518.1
Residential \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	178.7	164.8	439.8	411.5
Residential \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Commercial
Disaggregation of Revenue [Line Items]
Customer revenues	250.8	224.1	900.4	849.3
Commercial \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	99.2	91.8	496.0	483.9
Commercial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	151.6	132.3	404.4	365.4
Commercial \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Industrial
Disaggregation of Revenue [Line Items]
Customer revenues	166.7	145.5	510.6	466.7
Industrial \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	41.0	42.8	142.9	165.6
Industrial \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	125.7	102.7	367.7	301.1
Industrial \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Off-system
Disaggregation of Revenue [Line Items]
Customer revenues	14.5	6.0	46.0	32.7
Off-system \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	14.5	6.0	46.0	32.7
Off-system \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Off-system \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	0.0	0.0	0.0	0.0
Miscellaneous
Disaggregation of Revenue [Line Items]
Customer revenues	3.8	14.2	24.4	41.8
Miscellaneous \| Gas Distribution Operations
Disaggregation of Revenue [Line Items]
Customer revenues	4.6	6.8	19.2	24.6
Miscellaneous \| Electric Operations
Disaggregation of Revenue [Line Items]
Customer revenues	(1.0)	7.2	4.6	16.6
Miscellaneous \| Corporate and Other
Disaggregation of Revenue [Line Items]
Customer revenues	$ 0.2	$ 0.2	$ 0.6	$ 0.6